The present invention relates to a liquid crystal display apparatus having a plurality of display digits.
The electric power consumption of a liquid crystal display device is remarkably small as compared with a display devices using fluorescent display devices and light emitting diodes. Therefore, in the case of using the display device in conjunction with a microcomputer or the like, a liquid crystal display device having a small electric power consumption is advantageously used. Especially in the case where a battery is used as a power source, the use time of the power source is prolonged; therefore, the liquid crystal display device is even more advantageously used.
FIG. 1 shows a liquid crystal display apparatus using a one-chip microcomputer (.mu.PD 7514) 1 which is on the market from Nippon Electric Co., Ltd. (NEC) to dynamically drive the liquid crystal display device. This liquid crystal display apparatus comprises: four resistors 2 to 5 connected in series between a positive power source terminal VT and the ground; and capacitors 6 to 8 connected respectively between positive power source terminal VT and nodes N.sub.1 to N.sub.3 of resistors 2 to 5. Voltage source terminals VDD, VLC.sub.1, VLC.sub.2, and VLC.sub.3 of one-chip microcomputer 1 are connected to positive power source terminal VT and nodes N.sub.1 to N.sub.3. To reduce the electric power consumption, for example, each resistance value of resistors 2 to 4 is set to 100 k.OMEGA., a resistance value of resistor 5 is set to 82 k.OMEGA., and a voltage between voltage source terminals VDD and VLC.sub.1 is set to about 3 V. Microcomputer 1 drives the liquid crystal display device in accordance with a 1/3 bias system. Namely, in the case of energizing the segment, a common signal which varies from the VDD level to the VLC.sub.3 level as shown in FIG. 3A is applied to the common electrodes in each period T, and a segment signal which changes from the VLC.sub.3 level to the VDD level as shown in FIG. 3B is applied to the segment electrodes. In the ordinary operating mode, when no segment is energized, a common signal which varies from the VLC.sub.2 level to the VLC.sub.1 level as shown in FIG. 2A is applied to common electrodes in each period T, and a segment signal which changes from the VLC.sub.1 level to the VLC.sub.2 level as shown in FIG. 2B is applied to segment electrodes. On the contrary. Microcomputer 1 uses voltages which are applied to voltage source terminals VDD and VLC.sub.1 to VLC.sub.3 as voltages at the levels of VDD and VLC.sub.1 to VLC.sub.3.
In such a kind of liquid crystal display apparatus, suppose that the liquid crystal display device has a weight display section consisting of five digits including the minus sign, a unit price display section consisting of five digits, and a price display section consisting of six digits, for example. In this case, in the cases where, for example, "0" of one digit is displayed on each of the weight, unit price, and price display sections and where fifteen numerals are displayed on these display sections, there is the problem such that the luminances of the displayed liquid crystal display elements differ. For example, assume that a voltage VX between voltage source terminals VDD and VLC.sub.3 is set to 3 V in the case where eight numerals are displayed. Then, in the case of the display of three numerals, an effective impedance of the liquid crystal display device increases and a current flowing through resistor 5 decreases, so that voltage VX will increase to 3.3 V. In the case of the display of fifteen numerals, a large current flows through resistor 5, so that voltage VX will be reduced to 2.7 V. To properly drive the liquid crystal display device, voltage VX needs to be suppressed within a range of 2.9 to 3.1 V. In the case where numerals of fewer digits were displayed, these numerals become too bright. On the contrary, in the case where numerals of many digits were displayed, these numerals become faint. In this manner, the luminances of the liquid crystal display elements to be displayed largely vary in dependence on the number of digits of numerals to be displayed, so that the displayed numerals are fairly hard to see.